The original X-37A
was to feature an Rocketdyne AR-2-3 engine for maneuvering and deorbiting, which
uses Kerosene JP-8/Hydrogen Peroxide as fuel and oxidizer.
Later (X-37B) the propulsion system has been changed to a hypergolic Hydrazine/Nitrogen
Tetroxide engine (R-4D ?) with a robust propellant load for maneuvering in space
and for the deorbit burn.